The utility of fiber-reinforced plastic composites as a material for equipment components has been widely recognized. These componets are often lighter, less prone to corrosion, and less expensive than comparable parts made of metal. They are particularly suited for transportation and power generation equipment where their low density and high corrosion resistance are harnessed to make more energy efficient automobiles and lower maintainece wind turbines. However, making components out of fiber-reinforced plastic composites still has many challenges.
For components that are regularly exposed to severe weather, jaring vibrations, and significant impacts, toughness and repairability are an important concern. Fiber-reinforced composites made using traditional thermoset polymers are well regarded for their strength and corrosion resistance, but poorly regarded for being prone to cracking and shattering with little opportunity to make meanful repairs. When critical cracks or fractures are discovered in such a thermoset part, they normally must be replaced instead of repaired, and virtually no materials from the damaged part can be recycled into new components.
The shortcomings with fiber-reinforced composite parts made using thermoset plastics has prompted many industries to consider thermoplastic substitutes. Unlike thermoset polymers, thermoplastics are meltable, allowing cracks and breaks to be repaired, and recycleable when a component is beyond repair or has reached the end of its useful life. Thermoplastics can also be more easily engineered to give a part increased fracture toughness that reduces the frequency of cracks or breaks.
The benefits of fiber-reinforced thermoplastic composite parts are counterbalanced by increased difficulties in making components from thermoplastics. Traditionally thermoplastic composites are made by compounding thermoplastic polymer resins with chopped fibers and injection molding. However, the resulting thermoplastic composites have limited mechanical strength due to the short fiber lengths. The molten thermoplastic polymers are typically more viscous than the pre-reacted components used to make thermoset polymers, which makes them significantly more challenging to adequately impregnate continuous fibers such as woven fabrics. The high melt viscosity of the thermoplastic polymers prevents them from being used in conventional liquid molding processes, such as resin infusion processes and resin transfer molding processes that are often used to make structural parts from lower-viscosity thermoset resins. Thermoplastic polymers with high melting points also require high temperature molding equipment that can be difficult to operate and more prone to breakdown.
One approach to addressing the viscosity problems with thermoplastic polymer melts has been to use reactive thermoplastic resins that introduce the low-viscosity pre-polymerized reactants to the mold and have them polymerize in situ. In this way reactive thermoplastic resins can be processed using similar techniques as thermoset resins, but once polymerized give the component the repairability and recyclability properties of a conventional fiber-reinforced thermoplastic composite.
While reactive thermoplastic resins can solve the high viscosity problems experienced with conventional thermoplastic polymer melts, they have challenges of their own. Currently, there are fewer classes of reactive thermoplastic resins when compared to conventional thermoplastic polymer resins. Many reactive thermoplastic resins have their own challenges in the polymerization step. For example, caprolactam-based polyamide-6 resin has the advantages of water-like viscosity of raw material (molten caprolactam) and fast polymerization. However, the anionic polymerization of caprolactam to form polyamide-6 is very sensitive to moisture. Keeping a manufacturing system moisture-free is extremely challenging for processing large composite parts such as wind turbine blades. In another example, reactive thermoplastic resins that include methyl methacrylate (MMA) have to contend with highly exothermic polymerization reaction of MMA to form polymethyl methacrylate (PMMA). The heat released from the MMA polymerization reaction can quickly raise the temperature of the resin above the boiling point of the MMA (˜101° C.), causing many processing problems. These problems are especially severe for the manufacture of large composite parts that require large amounts of the reactive MMA resin. These and other challenges are addressed by the present invention.